The invention relates to the technical field of tribology in lubricated medium.
More particularly, the invention relates to coatings and surface treatments designed to reduce wear and to minimize the transmission of tangential forces.
Numerous technical solutions have been proposed for improving the tribological performance of mechanical parts. A distinction is made essentially between conventional case hardening treatments and deposits of thin and hard layers obtained by various methods such as PVD (Physical Vapour Deposition) or PACVD (Plasma Assisted Chemical Vapour Deposition).
Among the deposits of thin and hard layers, mention can be made of deposits of transition metal nitrides (TiN, CrN, TiAIN, etc.), coatings of amorphous carbon (DLC), etc. For technical, as well as mechanical reasons, it appears that these deposits of surface coatings do not generally exceed 5 μm. Above this thickness, risks of embrittlement of the layer and flaking may occur. It is also important to obtain perfect adhesion and strength of the thin layer over time. Thus, a person skilled in the art claims very slightly irregular surface textures with a roughness (Ra) of about 0.04 μm.
It therefore appears from the prior art that the tribological performance of the surface treatments and vacuum deposits are only guaranteed with surfaces having a slight roughness.
For example, mention can be made of the teaching of U.S. Pat. No. 6,886,521, which sets a maximum value of the surface roughness parameter (Rz) as a function of the hardness of the DLC deposit and the thickness thereof.
In the field of tribology, numerous investigations have been conducted on the influence of the topography of the contact surfaces on the establishment of a hydrodynamic lubrication regime. Thus, it has been proposed to carry out a texturizing of bearings or thrust bearings in order to promote the separation of the surfaces by an improved lubricant bearing capacity. Mention can be made, for example, of the teaching of documents U.S. Pat. No. 5,952,080 and WO2004/063533.
However, the dimensioning of the surface topography is not a simple matter, so that this solution is incompatible with severe contact conditions, and also in the case of heavily loaded friction parts, that is, where the contact pressures give rise to operation in conditions called elastohydrodynamic regime. In fact, this texturizing, which is equivalent to etching patterns on the contact surface, causes a significant decrease in the bearing surface area, so that an unsuitable texturizing inevitably causes a drop in pressure of the oil film and damage to the surfaces, which militates against the desired objective.
It follows that the principle of surface texturizing is not applied by a person skilled in the art except in cases of slightly loaded friction parts in lubricated medium.
Based on the analysis of this prior art, one of the problems that the invention proposes to solve is to be able to apply the principle of the texturizing of a friction surface in order to prolong the existence of an elastohydrodynamic lubrication regime between friction parts in lubricated and heavily loaded medium, that is, working under contact pressures, for example, above 200 MPa.
In fact, above a certain contact pressure threshold, the exponential increase in the viscosity of the lubricant (several orders of magnitude) radically alters its physical behaviour. The lubricant then changes state, and behaves in a manner closer to that of a solid than a fluid. The complete separation of the contact surfaces is then permitted by the elastic deformation of the opposing surfaces, under the action of the lubricant that has become extremely viscous. This creates the conditions called elastohydrodynamic regime. The physical behaviour of the lubricant and the opposing surfaces being radically different in elastohydrodynamic regime, thus explaining why the optimisation of a surface texturizing for contact takes place very differently than for the case of other lubricated interfaces.